1. Field of the Invention
The present invention relates to a condenser microphone of which the sensitivity can be adjusted, and particularly relates to a condenser microphone that include a plurality of condenser microphone units between which variations in sensitivity can be controlled.
2. Related Background Art
An electret condenser microphone unit includes an electret material, composed of a material such as fluoroethylene polymer (FEP), on at least one of a diaphragm and a fixed electrode opposed to the diaphragm. In a known method of forming a material into an electret, corona discharge is used, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 11-117172. A corotron or scorotron is typically used for generating corona discharge. A non-uniform electric field is formed in such a device so that corona discharge is generated therein. This generates negative ions to which the material is then exposed and formed into an electret. The electret material formed in such a way generally has a variable surface potential depending on the discharge condition. Such a variation in surface potential causes about 10% of variation in sensitivity in an electret condenser microphone unit including the electret material.
Some condenser microphones such as stereo condenser microphones and variable directional condenser microphones each include a plurality of condenser microphone units. Since variations in sensitivity between the condenser microphone units have a disadvantageous effect on the microphone, the sensitivity of each unit must be set constant. For example, in the case where the above-described electret condenser microphone units are included in such a condenser microphone, the condenser microphone units must have small variations in sensitivity therebetween or the sensitivities of the individual condenser microphone units must be adjusted. Much time and cost, however, are required for picking up units having small variations in sensitivity from a large number of condenser microphone units. In addition, no simple method has been proposed to control the sensitivities of individual condenser microphone units.
The sensitivity of the condenser microphone unit may vary with any factor other than a variation in surface potential of an electret layer. For example, the sensitivity of the condenser microphone unit may vary due to a structural relationship between components included in the unit. It is therefore desirable that the sensitivity be adjusted after the components are assembled into a condenser microphone unit.
Japanese Unexamined Patent Application Publication No. 2008-131160 discloses a condenser microphone that includes a needle-shaped electrode having a tip near the back of a fixed electrode of an electret condenser microphone unit, in which a voltage is applied to the needle-shaped electrode to generate ions so as to neutralize an electret and thus to adjust the sensitivity of the microphone unit. The sensitivity of this condenser microphone unit can be adjusted using the ions that neutralize the electret even after the microphone is assembled.
Unfortunately, in the case where a plurality of condenser microphone units are included in such a condenser microphone, each condenser microphone unit needs to have a needle-shaped electrode in order to adjust the sensitivity of the microphone unit, leading to a complicated configuration of the unit. Moreover, a plurality of needle-shaped electrodes must be provided, leading to an increase in manufacturing cost of the condenser microphone. Furthermore, JP-A-2008-131160 does not refer to the control of the sensitivities of two or more condenser microphone units.